1. Field of the Invention
The present invention relates to a process for producing a so-called longitudinal magnetic coated recording medium.
2. Description of the Prior Art
A magnetic recording medium comprises a magnetic film which is formed on a nonmagnetic substrate and contains magnetic particles dispersed therein. The easy magnetization axis of the magnetic particles may be oriented either in parallel to or perpendicular to the surface of the magnetic film. Acicular magnetic particles e.g., .gamma.-Fe.sub.2 O.sub.3 , are commonly used for dispersion in a magnetic film. Such magnetic particles have an easy magnetization axis in the direction of its longitudinal axis, and tend easily to orient in a direction in parallel to the surface of the magnetic film. Thus, their magnetic orientation is parallel to the surface of the magnetic film, hereinafter referred to as longitudinal orientation.
When information is written in and read out of a longitudinal magnetic film by means of a ring head, the magnetic fluxes which a detecting ring head bisects are almost parallel to the surface of the magnetic film, and the detected magnetic fluxes exhibit a normal distribution as shown by a broken line 1 in FIG. 1. On the other hand, when planar magnetic particles 4, each having an easy magnetization axis 5 perpendicular to the major surface thereof, (e.g., barium ferrite, such as BaFe.sub.12 O.sub.19 , Ba.sub.2 Me.sub.2 Fe.sub.12 O.sub.22 , Ba.sub.3 Me.sub.2 Fe.sub.24 O.sub.41 , Ba.sub.3 Co.sub.1.92 Fe.sub.0.08 Fe.sub.24 O.sub.41 as shown in FIG. 2), are dispersed in a magnetic film, the major surface of the planar magnetic particles tends to be arranged in parallel to the surface of an aluminum or polyester substrate particularly when the substrate is a disc which rotates during spin-coating. In this case, the easy magnetization axis 5 of the planar magnetic particle 4 is oriented perpendicular to the surface of the magnetic film.
In such a case, the detected fluxes exhibit an abnormal asymmetric distribution, as shown by a one-point broken line 2 in FIG. 1. Furthermore, a practical magnetic recording medium, which contains perpendicular barium ferrite particles, has a certain amount of longitudinal magnetic components. Thus, the broken line 1 and one-point broken line 2 are synthesized, and thereby the detected curve is deformed as shown by a solid line 3 in FIG. 1, and the peak of the detected flux is dislocated from the original point. It is necessary to pass the detected signal through a correction circuit which comprises a delay circuit, an attenuator, etc., prior to the writing and reading of information. In addition, the output and practical recoding density are inevitably low.
The hexagonal crystal platelets with easy magnetization axes normal to their planes such as barium ferrite BaFe.sub.12 O.sub.19 etc are recognized to be effective for improving the recording density. However, barium ferrite is not practically used to produce a perpendicular recording medium due to the above-mentioned defects. Furthermore a longitudinal magnetic coated recording medium comprising barium ferrite is not realized, because it is difficult to form a longitudinal magnetic film therefrom.
Wada et al, Japanese Unexamined Patent Publication No. 57-212623, assigned to Toshiba Denki K.K., discloses a magnetic recording medium which contains barium ferrite type magnetic particles with an average size of 0.3 .mu.m in an amount of at least 50% by weight of the total magnetic particles. Wada et al includes .gamma.-Fe.sub.2 O.sub.3 type or cobalt type magnetic particles with an average size of 0.4 .mu.m. However, Wada et al does not teach or suggest the application of a magnetic field prior to drying the magnetic paint.
Kishimoto et al, Japanese Unexamined Patent Publication No. 59-129933 assigned Hitachi Maxell discloses a magnetic recording medium which contains barium ferrite magnetic particles having an easy magnetization axis perpendicular to the major surface thereof and other magnetic particles having an isometric magnetic axis. However, Kishimoto et al teaches the application of a magnetic field perpendicular to the surface of the film base, thereby forming a perpendicular magnetic coated recording medium.
Taniguchi, Japanese Unexamined Patent Publication No. 52-141612, assigned to Hitachi Maxell K.K. discloses a process for producing a magnetic recording medium wherein the surface of a substrate is coated with a magnetic paint. The painted substrate is then passed through a magnetic field which arranges the magnetic particles (e.g., .gamma.-Fe.sub.2 O.sub.3), dispersed in the magnetic paint in parallel to the running direction of the substrate, and then the paint is dried. However, Taniguchi does not teach the use of planar magnetic particles having an easy magnetization axis perpendicular to the major surface thereof.
Ishida et al, Japanese Unexamined Patent Publication No. 54-9905, assigned Fujitsu K.K. discloses a process for producing a magnetic recording disc, comprising the steps of: forming a magnetic film containing isometric magnetic particles, i.e., .gamma.-Fe.sub.2 O.sub.3, on the disc, applying a magnetic field perpendicular to a surface of the disc, and then applying an intrafacially orienting magnetic field parallel to the running direction of the disc. However, Ishida et al does not teach or suggest the application of a longitudinal magnetic field solely, or alternately with a perpendicular magnetic field, to a planar magnetic powder having an easy magnetization axis perpendicular to the major surface thereof.
Yokoyama et al, Japanese Unexamined Patent Publication No. 58-6526, assigned Toshiba Denki K.K. discloses a magnetic recording medium produced by painting a substrate with a resinous compound comprising planar magnetic particles each having an easy magnetization axis perpendicular to the major surface of the particle (e.g., hexagonal ferrite crystals). The substrate is painted to a thickness of at 0.2 .mu.m maximum, and has a ratio of diametric size to thickness of at least 2, the particles being magnetically arranged in the moving direction of a magnetic head. However, Yokoyama et al does not teach the process whereby the planar magnetic particles are oriented, except that a magnetic field of 2 kOe is applied to a polyethylene film coated with the magnetic resin and calendered to form a recording tape having a ratio of residual magnetization to saturation magnetization of 0.9. Yokoyama et al does not refer to the orientation angle, which the inventors originally defined as an angle where the ratio of residual magnetization to saturation magnetization Mr/Ms is the largest. In addition, it seems that the ratio of residual magnetization to saturation magnetization is 0.9, which appears to be too large to be actually attained. Moreover, the magnetic medium with barium ferrite particles magnetically arranged in the moving direction of a magnetic head inevitably contains many pinholes, and, therefore, it can not be put into practical use.